1. Field of the Invention
The present invention relates generally to semiconductor processing equipment and specifically to systems and methods for supporting a substrate during material deposition processes.
2. Description of the Related Art
High-temperature ovens, or reactors, are used to process substrates for a variety of reasons. In the electronics industry, substrates such as semiconductor wafers are processed to form integrated circuits. A substrate, typically a circular silicon wafer, is placed on a substrate holder. If the substrate holder helps to attract radiant heat, it is called a susceptor. The substrate and substrate holder are enclosed in a reaction chamber, typically made of quartz, and heated to high temperatures by a plurality of radiant heat lamps placed around the quartz chamber. In an exemplary high temperature process, a reactant gas is passed over the heated substrate, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material onto a surface of the substrate. As used herein, the terms “processing gas,” “process gas,” and “reactant gas” generally refer to gases that contain substances to be deposited upon a substrate (such as silicon-containing gases). As used herein, these terms do not include cleaning gases. Through subsequent processes, the layers of reactant material deposited on the substrate are made into integrated circuits. The process gas flow over the substrate is often controlled to promote uniformity of deposition across the top or front side of the substrate. Deposition uniformity can be further promoted by rotating the substrate holder and wafer about a vertical center axis during deposition. As used herein, the “front side” of a substrate refers to the substrate's top surface, and the “backside” of a substrate refers to the substrate's bottom surface.
As mentioned above, a typical substrate to be processed comprises a thin layer of silicon. In the production of integrated circuits, it is sometimes desirable to deposit additional silicon (e.g., via CVD) onto the substrate surface(s). If the new silicon is deposited directly onto the silicon surface of the substrate, the newly deposited silicon maintains the crystalline structure of the substrate. This is known as epitaxial deposition. However, the surfaces of the original substrate to be processed are typically polished and naturally covered with a native oxide layer (e.g., SiO2). The deposition of new silicon onto the native oxide layer is known as polysilicon deposition. In order to conduct epitaxial deposition, it is ordinarily necessary to remove the native oxide layer from each surface (i.e., the substrate's top and/or bottom surfaces) onto which the new silicon is to be deposited. The native oxide layer is typically removed by exposing it to a cleaning gas, such as hydrogen gas (H2), prior to the deposition of new silicon. As used herein, the term “cleaning gas” is different than and does not encompass reactant gases.
There are a large variety of different types of substrate holders for supporting a substrate during processing. A typical substrate holder comprises a body with a generally horizontal upper surface that underlies the supported substrate. A spacer means is often provided for maintaining a small gap between the supported substrate and the horizontal upper surface. This gap prevents process gases from causing the substrate to stick to the substrate holder. The substrate holder often includes an annular shoulder that closely surrounds the supported substrate. One type of spacer means comprises a spacer element fixed with respect to the substrate holder body, such as an annular lip, a plurality of small spacer lips, spacer pins or nubs, etc. An alternative type of spacer element comprises a plurality of vertically movable lift pins that extend through the substrate holder body and are controlled to support the substrate above the upper surface of the substrate holder. Often, the spacer element is positioned to contact the substrate only within its “exclusion zone,” which is a radially outermost portion of the substrate within which it is difficult to maintain deposition uniformity. The exclusion zone is normally not used in the development of integrated circuits for commercial use. A processed substrate may be characterized, for example, as having an exclusion zone of 5 mm from its edge.
One problem associated with CVD is the phenomenon of “backside deposition.” Many substrate holders are unsealed at the substrate perimeter, so that process gases can flow down around the peripheral edge of the substrate and into the gap between the substrate and the substrate holder. These gases tend to deposit on the substrate backside, both as nodules and as an annular ring at or near the substrate edge. This creates non-uniformities in substrate thickness, generally detected by nanotopology tools. Such non-uniformities in substrate thickness can adversely affect, and in many cases make impossible, subsequent processing steps, such as photolithography.
Prior to epitaxial deposition, the front side of the substrate is typically exposed to a cleaning gas to remove the native oxide layer. However, the unsealed substrate perimeter permits the cleaning gas to contact the backside of the substrate, resulting in oxide removal on the backside. The amount of cleaning gas that contacts the substrate backside is ordinarily not sufficient to remove the entire oxide layer therefrom. However, the cleaning gas tends to create pinhole openings in the oxide layer on the substrate backside, exposing the silicon surface. In particular, the pinhole openings tend to form in an annular ring or “halo.” Of course, the longer the exposure to cleaning gas, the further inward the cleaning gas effuses toward the substrate's center, creating more pinhole openings in the oxide layer. Some of the removed oxide can redeposit onto the oxide layer of the substrate backside to form concentrated bumps of SiO2. Once deposition begins, the process gases can similarly effuse around the substrate edge from above the substrate. The partial native oxide removal can result in mixed deposition of process gas materials on the substrate backside—epitaxial deposition on the exposed silicon surfaces and polysilicon deposition on the oxide layer. The halo's intensity is sometimes increased because the concentrated SiO2 may receive non-depleted process gases. This can result in small polysilicon growths or bumps. These bumps of polysilicon scatter light and lead to a thick haziness. Thus, one way that backside deposition can be detected is to conduct epitaxial deposition and then look at the substrate backside for the presence of a halo or haziness.
One method to reduce backside deposition involves the use of a purge gas that flows upward around the wafer edge to reduce the downward flow of cleaning or process gases. For example, U.S. Pat. No. 6,113,702 to Halpin et al. discloses a two-piece susceptor supported by a hollow gas-conveying spider. The two pieces of the susceptor form gas flow passages therebetween. During deposition, an inert purge gas is conveyed upward through the spider into the passages formed in the susceptor, the purge gas flowing upward around the substrate edges to inhibit the flow of process gases to the substrate backside.
Another problem in semiconductor processing is known as “autodoping.” The formation of integrated circuits involves the deposition of dopant material (such as doped silicon) onto the front side of the substrate. Autodoping is the tendency of dopant atoms to diffuse downward through the substrate, emerge from the substrate backside, and then travel between the substrate and the substrate holder up around the substrate edge to redeposit onto the substrate front side, typically near the substrate edge. These redeposited dopant atoms can adversely affect the performance of the integrated circuits, particularly semiconductor dies from near the substrate edge. Autodoping tends to be more prevalent and problematic for higher-doped substrates.
One method of reducing autodoping involves a susceptor that has a plurality of holes that permit the flow of gas between the regions above and below the susceptor. Autodoping is reduced by directing a flow of gas horizontally underneath the susceptor. Some of the gas flows upward through the holes of the susceptor into a gap region between the susceptor and a substrate supported thereon. As diffused dopant atoms emerge at the substrate backside, they become swept away by the gas downward through the holes in the susceptor. In this way, the dopant atoms tend to get swept down into the region below the susceptor by a venturi effect. Exemplary references disclosing conventional substrate holders employing this method are U.S. Pat. No. 6,444,027 to Yang et al. and U.S. Pat. No. 6,596,095 to Ries et al.